The manufacture of submersible motors for pumps or the like is often complicated by two sometimes conflicting requirements. First, it is necessary to protect the motor against electical surges caused by lightning, power surges and switching surges. Such devices can be designed to protect against extremely high level surges, on the order of 2000 to 3000 volts or can be made more sensitive to protect against low level surges, on the order of 350 volts. Although external lightning arresters or surge protectors can be provided, for example, at the top of a well if the motor is to be used to pump water from a well, it is most desirable to provide the surge protection device inside the motor.
Counteracting the need for surge protection is the requirement that the motor be tested after complete assembly to determine if there are any weaknesses in the insulation system, lead wires or the like. Such a test, commonly referred to as a "hi-pot" test subjects the motor to high voltages defined as 1000 volts plus twice the line voltage for one minute or 1.2 times this voltage for one second. Thus, if the line voltage is 230 volts, for example, the motor would be subjected to 1460 volts for one minute or 1752 volts for one second.
The conflict between the two requirements should be apparent. If internal low level surge protection is desired, the hi-pot voltage would exceed the limit of the protection device and would go to ground, thus defeating the purpose of the test. Presently, the only internal devices used are for high level surge protection where the hi-pot voltage does not exceed the limit of the protection device. Usually these types of devices are air gap devices where the 2000 or 3000 volts will spark over to ground. However, not only are these devices limited to high level protection, as previously described, but they are also subject to life limitations in the number of spark-overs.